Problem: In math class, the girl to boy ratio is $10$ to $9$. If there are $36$ boys, how many girls are there?
Answer: A ratio of $10$ girls to $9$ boys means that there is a group of $10$ girls for every group of $9$ boys. If there are $36$ boys, then there are $4$ groups of $9$ boys each. Because we know the ratio is $10:9$ , there must be $4$ groups of $10$ girls each. There is a total of $40$ girls in math class.